1. Field of the Invention
The present invention relates to a MEMS (micro electro mechanical system) and a method for manufacturing thereof.
2. Description of the Related Art
Many electronic systems used in high frequency band are super-small, super-lightweight and high-powered. Accordingly, widely studied is a super-small micro-switch using a new technology named micro-machining to replace semiconductor switches such as FET (field effect transistor) or PIN diode used to control a signal in these systems.
The most manufactured RF (radio frequency) element using MEMS (micro-electro mechanical system) is a switch. The RF switch is often applied in an impedance matching circuit or a signal selection transmission at a wireless communication terminal or system in a microwave or millimeter wave band.
When DC (direct current) voltage is supplied to the fixing electrode, the conventional MEMS switch is charged between a fixing electrode and a moving electrode. The moving electrode is pulled towards a substrate by electrostatic force. After that, a contract member formed on the moving electrode is in contact with a signal line formed on the substrate, and switch is on or off.
An example on the above-mentioned MEMS switch is disclosed in U.S. Pat. No. 6,100,477.
FIG. 1 is a view of the structure of a MEMS (micro-electro mechanical system) switch in a prior art, showing the MEMS switch disclosed in the U.S. Pat. No. 6,100,477 in the off state. FIG. 2 shows the MEMS switch of FIG. 1 in the on state.
Referring to FIGS. 1 and 2, the MEMS switch in the prior art includes: a substrate 28 formed with a cavity 30; a fixing electrode 38 formed on at least one part of the cavity 30; a membrane 38 formed at an interval with the fixing electrode 38 and transformed towards the fixing electrode 34 as a voltage is supplied to the fixing electrode 38; and insulating layers 32, 40. The membrane 34 is provided with a bending structure 36 therearound to flexibly support the membrane 34.
The MEMS also includes a RF (radio frequency) inputting end 44, a DC (direct current) bias 42, a fixing capacitance 46 and a RF outputting end 48.
FIG. 3 is a view of a structure of another MEMS switch in the prior art, showing a structure of the MEMS switch disclosed in the U.S. patent application Publication No. US2003/0227361. FIG. 4 is a view taken along a line IV-IV of FIG. 3 showing a switch-off state, and FIG. 5 is a view taken along a line IV-IV of FIG. 3 showing a switch-on state.
Referring to FIGS. 3 through 5, a MEMS (micro electro mechanical system) switch 40 includes RF (radio frequency) conductors 42, 43 which are disposed on a substrate 44.
An upper part of the substrate 44 is provided with a bridge structure 46 having a central rigid body 48. The central rigid body 48 is vertically movable by spring arms 50 connected with supporting members 52.
The central rigid body 48 is formed with segments 54, 55, 56 on a center and edge parts. The bridge structure 46 is formed with the spring arms 50 which is, at one part, extended along the underside of the central rigid body 48. The spring arms 50 form electrode portions 60, 61, respectively. The segment 56 is provided with a contact member 64 electrically connecting the RF conductors 42, 43, when the switch 40 operates.
The electrode portions 60, 61 are supported by the supporting members 52.
The substrate 44 is formed with electrodes 70, 71 corresponding to the electrode portions 60, 61. Both sides of the electrodes 70, 71 are provided with stoppers 74, 75 restricting a descending operation of the central rigid body 48.
However, the abovementioned switches in the prior art are formed with the membrane in contact with the entire surface of the contact member 64, easily causing a sticking failure and accordingly lowering reliability.
The switching operation occurs in the central part of the membrane 34 in FIGS. 1 and 2 or the central part of the central rigid body 48 in FIGS. 3-5, which have relatively less restoring force than other portions therearound, easily causing the sticking failure.
When the membrane 34 or the central rigid body 48 is moved downward, the abovementioned MEMS switch decreases the restoring force and accordingly causing aggravated stability due to the sticking failure.